/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Mechanics/CubeGrid.h
Convert face direction to the index of the nonzero normal dimension, or equivalently, the dimension in which that face is flat.
static int8 DirToFlatDim ( [EFaceDirection](API\Plugins\ModelingComponents\Mechanics\FCubeGrid\EFaceDirection) DirectionIn )